For industrial PSA tape applications it is very common to use polyacrylate PSAs. Polyacrylates possess a variety of advantages over other elastomers. They are highly stable toward UV light, oxygen, and ozone. In contrast, synthetic and natural rubber adhesives normally contain double bonds, which make these adhesives unstable to the aforementioned environmental influences.
Another advantage of polyacrylates is their transparency and their usefulness across a relatively wide temperature range.
Polyacrylate PSAs are generally prepared in solution by free radical polymerization. The polyacrylates are generally coated onto the corresponding backing material from solution, using a coating bar, and then dried. In order to increase the cohesion the polymer is crosslinked. Curing proceeds thermally or by UV crosslinking or by EB curing (EB: electron beams). The operation described is relatively costly and environmentally objectionable, since as a general rule the solvent is not recycled and the high consumption of organic solvents represents a high environmental burden.
It is very difficult, moreover, to produce PSA tapes at high coatweight without bubbles.
One remedy for these disadvantages is the hotmelt process. In this process the PSA is applied to the backing material from the melt.
However, this technology has its limitations. Prior to coating, the solvent is removed from the PSA in a drying extruder. The drying operation is associated with a relatively high temperature and shearing effect, so that high molecular weight polyacrylate PSAs in particular are severely damaged. The acrylate PSA gels or the low molecular weight fraction is highly enriched as a result of molecular weight breakdown. Both effects are unwanted, since they are disadvantageous for the application. Either the adhesive can no longer be applied or there are changes in its adhesive properties, since, for example, when a shearing force acts on the adhesive the low molecular weight fractions act as lubricants and so lead to premature failure of the adhesive.
One solution to mitigating these disadvantages is offered by polyacrylate adhesives with a lower average molecular weight and narrow molecular weight distribution. In this case the fraction of low molecular weight and high molecular weight molecules in the polymer is greatly reduced as a result of the polymerization process. The fall in the high molecular weight fractions reduces the flow viscosity, and the adhesive shows less of a tendency to gel. As a result of the lowering of the low molecular weight fraction, the number of oligomers, which reduce the shear strength of the PSA, is lessened.
A variety of polymerization methods are suitable for preparing low molecular weight PSAs. The state of the art is to use regulators, such as alcohols or thiols, for example (Makromoleküle, Hans-Georg Elias, 5th edition, 1990, Hüthig & Wepf Verlag Basle). These regulators reduce the molecular weight but broaden the molecular weight distribution.
Under prolonged thermal exposure, such as is typical of a hotmelt process, however, volatile thiols are released as a result of thermal degradation or shearing, and these thiols have an unpleasant odor.
In the aforementioned publications attempts have been made to improve the control of radical polymerization reactions. There is nevertheless a need for a polymerization process which is highly reactive and allows high conversions to be obtained in conjunction with high molecular weight and low polydispersity. The task of meeting these requirements was taken on in DE 100 36 801.
A further variant for the preparation of polyacrylates is the RAFT (reversible addition-fragmentation chain transfer) process. The process is described at length in WO 9801478 and WO 9931144, but is not suitable in the manner depicted therein for preparing PSAs, since the conversions achieved are very low and the average molecular weight of the polymers prepared is too low for acrylate PSAs. The polymers described cannot, therefore, be employed as acrylate PSAs. An improvement was achieved with the process described in DE 100 30 217.3.
For these processes, however, there continues to be the problem of the odor nuisance caused by volatile thiol compounds, which are still formed by way of thermal decomposition or the effect of shearing.
Furthermore, for the preparation of acrylate PSA tapes, it is generally necessary to crosslink polyacrylates prepared by the above-described processes, using high-energy (actinic) radiation. This process as well (especially as a result of irradiation with electron beams) produces fragments which are composed of thiols and give rise to an unpleasant odor.
The problem of odor nuisance after thermal storage, under shearing, and under actinic radiation in acrylate hotmelt PSAs therefore remains, and therefore closes off a broad application field to the PSAs prepared as described above.
It is an object of the invention, therefore, to provide a process which provides PSAs which in the hotmelt process exhibit a reduced odor behavior, or none at all, under thermal storage, under shearing, and under actinic radiation.